The physiological effects of sound energy have been appreciated for some time. However, it was not until the passage of the Walsh-Healey Act that quantitative limits have been set on the permissible exposure of the human ear to sound or noise. Accordingly, there have been substantial efforts to provide an instrument which would help industry to determine their degree of conformance to the quantitative noise requirements of the Walsh-Healey Act.
As sound waves travel they radiate outward from their source. As the waves cover an increasingly large area, the strength thereof diminishes. A good rule of thumb is that the amplitude of waves are reduced by one-half when the distance is doubled, assuming of course that the sound is radiating from a relatively small source compared to the distance from the source. In addition, the human ear hears without damage pressure levels that are approximately 100,000 times stronger than the lowest pressure level that it can detect. Because the ear is sensitive to differentials in sound intensity, a sound reference unit was developed termed the decibel (dB). A decibel is mathematically defined as: ##EQU1## where A.sub.1 is the lowest pressure level that the human ear can detect and A.sub.2 is the pressure level measured. Thus, one dB corresponds to a pressure level ratio of 1.12:1, 6 dB corresponds to a pressure level ratio of 2:1 and 40 dB corresponds to a pressure level ratio of 100:1. A sound intensity of 130 dB is usually considered the threshold of pain.
It is well-known that sound intensity and loudness differ because the human ear is more sensitive to certain frequencies of sound than to others. Thus, a tone at 5000 Hz will be much louder than a tone of 100 Hz even though both are transmitted with the same sound pressure. A system for measuring loudness, that is, sound intensity as measured by the human ear, was developed by weighting the intensity of sound in accordance with the frequency thereof. Thus, a commonly designated "A" weighted filter has been developed which accomplishes this by providing a frequency response which approximates the hearing response of the human ear.
Noise limits as established by the U.S. Department of Labor under the Walsh-Healey Act use the A weighted decibel (dBA) scale to define the noise limits for industrial environments. These limits are derived from statistical studies of hearing losses and are set forth as follows:
TABLE 1 ______________________________________ Duration of Allowable Daily Exposure Level ______________________________________ Hours dB (A) 8 90 6 92 4 95 3 97 2 100 11/2 102 1 105 1/2 110 1/4 115 ______________________________________
A graphical relationship of the permissible human exposure time in hours per day vs. sound level in dB(A) as set out in Table 1 is shown graphically in FIG. 1 wherein the exposure time is represented by the ordinate and the sound pressure level as measured by the human ear is represented by the abscissa. From FIG. 1, a person could be exposed to a maximum of eight hours of noise at a 90 dB(A) pressure level. However, if the noise level increases to 95 decibels, it can be seen that a person can be exposed thereto for only 4 hours before the limits of the Walsh-Healey Act are exceeded. When the noise level rises to 115 dB(A), the total time exposure is only one-fourth of an hour.
Under actual working conditions, however, a person may be exposed to varying levels of sound pressure. Thus, for example, in one 8 hour day a person may be exposed to 2 hours of noise at 90 dB(A), 1 hour of 95 dB(A) noise and one-half hour of 100 dB(A) noise with the remaining noise exposure time being below 90 dB(A). In this noise environment, the exposure to 2 hours of 90 dB(A) noise accounts for 25% of the maximum allowable noise exposure for the day. The 1 hour of 95 dB(A) exposure accounts for 25% of the maximum allowable noise exposure and the one-half hour of 100 dB(A) noise exposure accounts for 25% of the maximum allowable noise exposure. Accordingly, during the aforementioned 8 hour exposure period, the person was exposed to 75% of the maximum allowable noise exposure.
In order to determine the exposure of an individual to noise, a portable audio dosimeter worn by the individual during the entire exposure period must be provided. There have been a number of such dosimeters provided in the past. For example, in U.S. Pat. No. 3,144,089, issued to Lane et al., there is shown a noise exposure meter wherein noise is detected by a microphone and converted to an electrical signal. The signal is rectified and then coupled to an electro-chemical integrator, such as a coulometer. The coulometer integrates the current passing therethrough so that the position of the gap of the coulometer is representative of the total noise exposure of the person wearing the meter. This dosimeter, however, does not take into account the sensitivity of the ear to various frequencies of the sound spectum and includes no means for determining whether the accumulated noise over a period of time is in compliance with the Walsh-Healey Act.
U.S. Pat. No. 2,884,085 to Wolf-Wito von Wittern et al. illustrates another example of past efforts to provide noise exposure meters. This patent discloses a microphone for detecting sound pressure levels and for converting the pressure levels to a corresponding electrical signal. An intensity discriminator is provided which consists of a plurality of vacuum tubes, each of which is gated at successively higher discrete voltage signal levels. As each vacuum tube is gated on, a relay is actuated to initiate operation of a clock. Accordingly, at a first noise level the first vacuum tube is gated and the clock associated therewith starts to operate. As the noise intensity increases, the second vacuum tube is gated to thereby initiate operation of a second clock associated therewith and so on until the noise intensity reaches such a level that each of the vacuum tubes are gated and each of the clocks are operating. After a predetermined period of time, the noise exposure at each sound level can be determined by reading the clocks. However, there is no means for taking into account the ear's sensitivity to different frequencies of sound and in addition a plurality of relay operated clocks are required which operate at relatively high levels of power. Thus this noise exposure meter could not be readily carried about by a person on the job for extended periods of time.
U.S. Pat. No. 3,696,202 issued to Ida et al. discloses an audio dosimeter which detects sound pressure levels and converts these levels to a weighted signal representing the loudness of the sound as detected by the human ear. This signal is then amplified and coupled to an electro-chemical device, such as a coulometer, for detection of the accumulated noise intensity level over a period of time. This patent, however, does not disclose a well-defined means for preventing noise levels lower than 90 dB(A) from being accumulated and stored by the coulometer. Accordingly, the coulometer will provide an output which indicates that the accumulated noise level is higher than it actually is. In addition, accurate response to peak noise levels is not provided since peak storage capacitors store the peak noise levels thereby distorting the time variance of the noise pressure signal. Accordingly, the coulometer provides a readout which once again is higher than the actual accumulated noise level.
Stevens et al. U.S. Pat. No. 3,697,973 discloses yet another dosimeter which is carried about in a relatively large case. The detected sound pressure level is appropriately filtered by an A weighted filter, squared by a square law detector and then converted to a pulse train by means of unijunction pulser. Because of the square law detector, the output of the Stevens et al dosimeter is not an accurate integration of the noise level input and the use of a unijunction pulser subjects the dosimeter to possible error due to drift within the electronic circuitry.
Other devices are available, such as disclosed in U.S. Pat. No. 3,594,506 issued to Bauer et al., which measure the loudness of sound waves but do not provide an indication of the quantity of noise to which a person is subjected and does not indicate compliance with the Walsh-Healey Act.
From the foregoing it can be seen that there is a substantial need for an accurate, portable audio dosimeter for detecting accumulated noise with respect to time so that compliance with the Walsh-Healey Act can be ascertained.